Tail call optimization (TCO) not working in Safari

Question

According to ES6 compatibility table, Safari has tail call optimization feature. Tried it and it fails just like any other browser 😭. Am I missing something?

function factorial(n, r = 1n) {
  return (n <= 1) ? r : factorial(n - 1n, n * r)
}

console.log(factorial(36000n))

Safari output:

RangeError: Maximum call stack size exceeded.

Answer 1

You need to run your program in strict mode.

"use strict";

function factorial(n, r = 1n) {
    return n <= 1n ? r : factorial(n - 1n, n * r);
}

console.log(factorial(36000n).toString());

There are four conditions that need to be met in order for a function call to be considered a proper tail call.

• The calling function is in strict mode.
• The calling function is either a normal function or an arrow function.
• The calling function is not a generator function.
• The return value of the called function is returned by the calling function.

Source: ECMAScript 6 Proper Tail Calls in WebKit by Michael Saboff